This conventional type of rear derailleur, is described in Japanese Patent Laid-Open Gazette No. Sho 59-156,882. In this type of derailleur, the support member at the linkage mechanism is supported swingably to the base member through a first horizontal shaft, the chain guide is supported swingably to the movable member at the linkage mechanism through a second horizontal shaft. Also, there is provided between the base member and the linkage mechanism a first spring and between the movable member and the chain guide a second spring, so that the springs provide a predetermined tension to the driving chain and balance with each other to set the guide sprocket in a radial position of each rear chain gear at the multistage rear chain gear assembly. The first spring biases the linkage mechanism clockwise with respect to the base member when viewed from the front of the derailleur, in other words, in the direction of moving the guide sprocket radially outwardly of the rear chain gear and away from the axis thereof. The second spring biases the chain guide clockwise with respect to the linkage mechanism when viewed from the front of the derailleur, in other words, in the direction of moving the guide sprocket radially inwardly of the rear chain gear and toward the axis thereof.
The well-known type of chain guide provided at the rear derailleur come within two general types. Namely, one chain guide, as described in the aforesaid Japanese Patent Laid-Open Gazette, has a guide sprocket and a tension sprocket, which are disposed together with the second horizontal shaft in a triangle, thereby being called the triangular balance type or the balance type chain guide (to be hereinafter referred to generally as the triangular balance type). The other type of chain guide, as described in Japanese Patent Laid-Open Gazette No. Sho 58-149,882, has its guide sprocket supported on the second horizontal shaft and the tension sprocket shifted therefrom, thereby being called the pendulum type chain guide.
In the rear derailleur provided with the triangular balance type or the pendulum type chain guide, the spring forces of the first and second springs are set to obtain the optimum distance between the guide sprocket and the smallest diameter rear chain gear at the multistage rear chain gear assembly when the driving chain engages with the smallest diameter rear chain gear and the chain tension is reduced to a minimum, or between the guide sprocket and the largest diameter chain gear at the gear assembly when the chain engages with the largest diameter rear chain gear.
Where the rear derailleur sets the first and second springs to obtain the optimum distance between the guide sprocket and the smallest diameter rear chain gear, the chain, when shifted to the larger diameter rear chain gear, increases in tension, whereby the chain guide swings largely counterclockwise with respect to the linkage mechanism against the second spring and the linkage mechanism swings counterclockwise with respect to the base member against the first spring. In this case, the second spring deflects more than the first spring so that the first and second springs balance with each other at the position where the chain guide swings over a predetermined angle with respect to the linkage mechanism. Hence, the guide sprocket moves away from the larger diameter rear chain gear and the distance between the guide sprocket and the larger diameter rear chain gear becomes larger than the predetermined range, thereby creating a problem in that the speed change efficiency is decreased.
Also, there is a problem in that the guide sprocket, according to its position, may move excessively toward the larger diameter rear chain gear to cause interference therewith.
Where the spring forces of the first and second springs are set to obtain the optimum distance between the guide sprocket and the largest diameter rear chain gear, the chain tension decreases when the chain is shifted to the smaller diameter rear chain gear, and the chain guide swings clockwise by the second spring, so that the second spring has its spring force reduced relative to the first spring so as to balance the first and second springs with each other in the condition where the linkage mechanism swings clockwise by means of the first spring. As a result, the guide sprocket moves excessively toward the smaller diameter rear chain gear, thereby creating a problem in that the speed change efficiency is diminished and occasionally both the guide sprocket and the rear chain gear interfere with each other.
In addition, there is the problem that the guide sprocket, according to its position, may move away from the smaller diameter rear chain gear with the result that a distance therebetween increases to lower the speed change efficiency.